Tire for motor vehicle wheels having a tread producing a low rolling noise

ABSTRACT

A tread for motor-vehicle tires has a raised pattern formed of a plurality of shaped blocks  7  distributed in circumferential rows  3, 4, 5, 6  disposed in parallel and in side by side relation. The number of the shaped blocks  7  arranged in each row gradually increases while moving from the equatorial plane X-X to the axial outer side edges  1   a  of the tread  1.  Thus a gradual increase in the longitudinal stiffness of the shaped blocks  7  is achieved on passing from the side edges  1   a  of the tread  1  to the equatorial plane X-X, which brings about a reduction in the mobility of said blocks on the road contact patch and lower rolling noise.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a tire for motor vehicle wheelshaving a tread producing a low rolling noise, which comprises a raisedpattern formed of a plurality of shaped blocks distributed in parallelcircumferential rows bounded by longitudinal grooves extendingcircumferentially around the tire with each row comprising a givennumber of shaped blocks separated from each other by respectivetransverse generally axially expanding cuts.

[0002] It is known that tires for road motor vehicles have a pluralityof longitudinal grooves and generally axially extending transverse cutson the tread, which define a plurality of shaped blocks thereon, saidblocks being distributed according to a carefully determined specificpattern.

[0003] Within the tread pattern in which the longitudinal grooves arepositioned the shaped blocks are distributed in several side by siderows. A proper distribution gives the tire the desired features ofdirectional control stability and road holding in relation to thesidewise thrusts directed parallel to the wheel axis. The transversecuts, in turn, give the tire the desired road traction, that is thecapability of efficiently transmitting tangential thrusts parallel tothe running direction during speeding up and slowing down of thevehicle. In principle, the traction of the tire tends to become greateras the number of the transverse cuts arranged in the tread increases andthe orientation of the cuts themselves approach an orientation which isperpendicular to the running direction.

[0004] In addition, the longitudinal grooves and transverse cutscooperate in performing an efficient draining action of water from thetread pattern area of the tire during running oil a wet road bed.

[0005] The presence of the transverse cuts and longitudinal grooveshowever causes the well known effect of noise produced by the rollingtire. One of the main causes of rolling noise has been found to becaused by the continuous succession of impacts of the shaped blockcorners on the road bed.

[0006] Another contributory cause in generating noise is the rubbingthat the shaped blocks undergo in contact with the road bed when theyenter and leave the ground contact patch. These rubbings are essentiallydue to the deformations necessarily undergone by the tread, when itsouter surface, which in a free state has a convex conformation,inevitably becomes flat against the road bed.

[0007] The dynamic actions causing the deformation of the tread betweenthe entry and exit regions to and from the contact patch also give riseto a cyclic variation of the volume of the grooves and cuts definedbetween the shaped blocks, which brings about a cyclic compressive anddecompressive action on the air contained in the grooves and cuts. Thisphenomena of air compression and decompression assist in the generationof noise produced by the rolling tire.

[0008] The known art suggests several expedients for limiting as much aspossible the noise produced by a tire rolling.

[0009] One expedient which has been adopted for a long time essentiallyconsists of differentiating the longitudinal dimensions, that is thosealong the circumferential extension, of the single shaped blocks. Inother words, the blocks are disposed on the circumferential extension ofthe tread according to two or more differentiated pitch valuesdistributed in a circumferential succession, usually referred to as“pitch sequence” which is generally carefully laid out so as to achievethe maximum possible avoidance of pattern repetition around the tirecircumference. In this manner the pressure waves generated by theimpacts and rubbings of the blocks do not take place according to aspecific frequency on which most of the acoustic energy generated as aresult of rolling would be concentrated. In other words, the acousticenergy is distributed over a wide frequency range, thereby eliminatingthe troublesome effect of harmonic repetition and noise usually known as“squirrel effect”.

[0010] For the purpose of distributing the acoustic energy over agreater number of frequencies, it has also been suggested to carry outoffsetting, with respect to each other, of the blocks belonging tocircumferential rows disposed consecutively in side by side relation.This solution however makes it difficult to drain the water from thecenter of the contact area towards the outer edges of the tread andtherefore makes it necessary to increase the width of the longitudinalgrooves for achieving a sufficient draining action.

[0011] Another expedient consists in giving the transverse cuts asuitably inclined orientation with respect to tire's axis of rotation.This enables the impacts of the block corners on the road bed to be“smoothed” but, on the other hand, it impairs the road traction of thetire.

[0012] A considerable reduction of the noise produced by rolling hasbeen achieved by forming additional narrow cuts usually referred to as“sipes” in the shaped blocks, said sipes acting to increase the elasticdeformability of the blocks. This solution too enables the impact of theblock on the road bed to be smoothed, thereby causing a reduction of thenoise resulting from impacts. However a greater deformability of a blockwill tend to increase the noise effects resulting from rubbings.

[0013] By way of example of the above described state of the art,reference is made to Assignee's prior U.S. Pat. No. 5,287,905 thedisclosure of which is hereby incorporated by reference.

SUMMARY OF THE INVENTION

[0014] In accordance with the present invention it has been found thatby arranging in the circumferential rows close to the equatorial plan(mid-circumferential plane) of the tire or center rows, a smaller numberof shaped blocks than is provided in the rows adjacent the axial outerside edges of the tread or shoulder rows, it is possible to achieve animportant reduction of the noise produced by rolling of the tire withoutimpairing the traction and direction control features of the presenttire and its efficiency in water draining.

[0015] In particular, the invention relates to a tire for vehiclewheels, in particular motor vehicle wheels, provided with a treadproducing a low rolling noise, characterized in that each of the axiallyouter rows have a greater number of shaped blocks than the number ofshaped blocks present in the axial inner adjacent row, that is the rowdisposed consecutively in side by side relation therewith towards theequatorial plane of the tread. This construction gives the tread agradually increasing longitudinal stiffness from the axially outer sideedges thereof towards the equatorial plane.

[0016] Close to the equatorial plane at least one continuouscircumferential rib may be disposed, said rib being defined between twoof said longitudinal grooves. Preferably, two of said circumferentialribs are provided and they are symmetrically disposed with respect tothe equatorial plane of the tread and mutually separated by a centralgroove.

[0017] According to a further feature of the invention, in each row ofsaid shaped blocks the transverse cuts define an impact angle with theadjacent longitudinal grooves which has a greater width than the impactangle formed with the transverse cuts of the block row disposedconsecutively in side by side relation therewith towards the equatorialplane.

[0018] In greater detail, the value of the impact angle in the blockrows that are closest to the equatorial plane is in the range of 15° to25°, whereas in the block rows closest to the axial outer side edges ofthe tread said impact angle is in the range of 70° to 105°.

[0019] Advantageously, each of said rows has first transverse cuts eachextending consecutively of one of the first transverse cuts belonging tothe block row which is disposed adjacent in side by side relation.

[0020] Said first cuts consecutively aligned define continuoustransverse grooves each extending from the corresponding axial outerside edge of the tread and toward said equatorial plane.

[0021] Each of said transverse grooves, starting from the correspondingaxial outer side edge has one straight portion substantiallyperpendicular to the longitudinal grooves, a curved union portion and asecond straight portion disposed obliquely relative to said longitudinalgrooves.

[0022] In addition, at least one of said rows of blocks has secondtransverse cuts each of them being intermediate between two of saidfirst cuts.

[0023] Preferentially, the second cuts are associated with outershoulder rows, adjacent to the outer side edges of the tread, and innershoulder rows disposed side by side with the outer shoulder rows towardsthe equatorial plane.

[0024] Each of said second cuts is spaced apart a varying distance fromthe first cuts contiguous therewith and has a smaller width than thefirst cuts. In particular, the ratio between the width of the secondcuts and that of the first cuts is in the range of 50% to 100%.

[0025] It is also provided that the blocks of at least one of said rowsshould each have at least one transverse slot extending towards saidequatorial plane over a portion of lower width than the width of theblock itself, as an extension of one of said second cuts. Saidtransverse slots are preferentially associated with outer center rowsdisposed consecutively and side by side with the inner shoulder rowstowards the equatorial plane of the tread.

[0026] Each of said transverse slots extends over about half the widthof the respective block.

[0027] At least one of said block rows further comprises third cutsextending parallel to said first and second transverse cuts. In greaterdetail, said third transverse cuts are associated with outer shoulderrows adjacent to the axial outer side edges of the tread.

[0028] Advantageously, the longitudinal grooves separating the outershoulder rows from the block rows contiguous thereto are interrupted inthe portions included between each of the third cuts and one of thefirst and second contiguous cuts.

[0029] The third cuts preferentially have a smaller width than saidfirst and second transverse cuts. In greater detail, along the extensionof each of said third cuts there is defined an inner portion facing saidequatorial plane and having a width lower than 1 mm and an outer portionof greater width than the inner portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Further features and advantages will become more apparent fromthe detailed description of a preferred embodiment of a tread patternproducing low rolling noise, in particular for motor vehicle tires,according to the present invention, given hereinafter by way ofnon-limiting example with reference to the accompanying drawings, inwhich:

[0031]FIG. 1 is a plan view of a tread portion in accordance with thepresent invention;

[0032]FIG. 2 is a graph showing the variation in the longitudinalstiffness of the tread of FIG. 1 at different points along itstransverse extension;

[0033]FIG. 3 is a diagram showing the variation in the number of shapedblocks provided in the different circumferential rows disposedconsecutively in side by side relation along the transverse extension ofthe tread.

DETAILED DESCRIPTION OF THE DRAWINGS

[0034] Referring to the drawings, a tread producing a low rolling noisein particular for motor vehicle tires in accordance with the presentinvention is generally identified in FIG. 1 by reference numeral 1.

[0035] The tread 1 conventionally has a plurality of longitudinalgrooves 2, spaced apart a suitable distance from each other andextending in a circumferential direction relative to the tire with whichthe tread is associated. Such grooves 2 define on the width of the tread1, a plurality of circumferential rows 3, 4, 5, 6 disposed parallel andin side by side relation and extending circumferentially around thetire, each comprising a predetermined number of shaped blocks 7, 7 a, 7b separated from each other by respective transverse cuts suitablyoriented relative to the longitudinal grooves 2.

[0036] In accordance with the present invention, advantageously in eachof the circumferential rows 3, 4, 5, 6 the number of shaped blocks 7, 7a, 7 b in greater than the number of blocks present in the adjacent rowtowards the equatorial plan X-X of the tread 1 (as denoted by line X-Xin FIG. 1). In other words, the number of blocks 7, 7 a, 7 b present ineach row 3, 4, 5, 6 gradually increases on passing axially outward fromthe central area toward either of the side edges 1 a of the tread 1.

[0037] This construction results in the tread 1 having a longitudinalstiffness which gradually increases from the outer side edges 1 athereof towards the equatorial plane X-X, for purposes to be laterclarified.

[0038] The Applicant has observed that this variation in the number ofblocks present in adjacent rows produces considerable effects in thefinished tire; especially during its uses when the number of rows ofblocks on each side of the center line of the tire is not lower than 3and preferably not higher than 5.

[0039] The particular embodiment shown in a prototype tire having aP-metric trade identification of 205/50-R-15, and is similar to priorart tires of the same size, e.g. Pirelli P700Z, in all usual respects(carcass structure, hardness of materials, inflation shape and pressure,depth and width of grooves, cuts and sipes, etc.) except for the hereindescribed tread pattern. In the present invention at the equatorialplane X-X of the tread 1, the number of blocks 7 is substantially zero,or, stated another way there is at least one circumferential continuousrib 11 located along plane X-X, that rib being defined between two ofsaid longitudinal grooves 2. Thus, one could consider continuous rib 11to be a single block, but in the present context where a “block” isdefined by transverse cuts, it is preferable to consider rib 11 to bezero “blocks”. In particular, the presence of two equatorial plane X-Xis preferably provided, said ribs preferably being mutually separated bya center groove 12 having substantially the same width as the individualribs 11. Obviously, in larger tires as the axial width of the tireincreases, there may be also an increase in the number of said ribs,although preferably said number would not be higher than three.

[0040] Externally of each rib 11, that is moving axially away in eachdirection from the equatorial plane X-X, there is an inner central row3, an outer central row 4, an inner shoulder row 5 and an outer shoulderrow 6, said rows being adjacently disposed in consecutively side by siderelation.

[0041] Each of said rows 3, 4, 5, 6 has first transverse cuts 8 each ofwhich, advantageously, is a continuation of one of the first transversecuts 8 belonging to the adjacent row or rows of shaped blocks which isadjacent in side by side relation therewith.

[0042] Practically, the first transverse cuts 8 as a whole definecontinuous transverse grooves 13 each extending from the correspondingouter side edge is of the trend 1 to an axial inward position close tothe equatorial plane X-X and more particularly close to one of thecircumferential ribs 11. This construction advantageously enables thewidth of the longitudinal grooves to be limited to increase traction ofthe tire. This is because the water draining action during running on awet road bed relies on the transverse grooves 13 which act as extensionswhich are substantially devoid of interruptions thus allowing the waterto easily flow towards the outer side edges of the tread 1.

[0043] In this connection, as can be seen from FIG. 1, the longitudinalgrooves 2 have a smaller width than the first transverse cuts 8 and thetransverse grooves 13 which are generated by the transverse cuts 8. Ingreater detail the ratio between the width “l” of the longitudinalgrooves 2 and the width “L” of the first transverse cuts 8 is between0.6 and 0.2 and preferably is about 0.4.

[0044] In addition, each of the transverse grooves 13 starting from thecorresponding side edge 1 a preferably should have one straight portionsubstantially perpendicular to the longitudinal grooves 2 and extendingclose to the respective shoulder rows 5, 6, one curved union portionextending close to the outer central row 4 and a second straight portiondisposed obliquely of the longitudinal grooves 2 and located close tothe inner central row 3.

[0045] As regards said straight portion, in its axially outermostsection, that is the axial outer section extending towards the tiresidewall, may lie at an inclination greater than 90° relative to theequatorial plane of the tire, this inclination preferably should not behigher than 105°.

[0046] In conclusion, due to the arrangement of the transverse grooves13, in each block row 3, 4, 5, 6 the transverse cuts 8 form with theadjacent longitudinal grooves 2, an impact angle “α” the amplitude ofwhich is greater than that of the impact angle formed with thetransverse cuts 8 of the block row disposed consecutively in side byside relation towards the equatorial plane X-X.

[0047] In other words, the impact angle “α” of the blocks 7 present inthe different rows 3, 4, 5, 6 gradually increases from the equatorialplane X-X towards the outer side edges 1 a of the tread.

[0048] In more detail, the value of the impact angle “α” in the innercentral rows 3 closest to the equatorial plane X-X is preferably in therange of 15° to 25°, whereas the value of the impact angle “α” in theouter shoulder rows 6 closest to the side edges 1 a of the tread 1 ispreferable in the range of 70° to 105°.

[0049] In the embodiment shown, the value of the impact angle αgradually increases moving away from the equatorial plane X-X, from 20°at the inner center rows 3, to 90° at the outer 6 and inner 5 shoulderrows.

[0050] In addition one or more of the block rows, (in the embodimentshown the outer 6 and inner 5 shoulder rows) preferably are providedwith second transverse cuts 9 each of them being intermediate betweenthe first cuts contiguous thereto and spaced apart a varying distancetherefrom.

[0051] This varying distance enables suitable pitch sequences, which areas much as possible uneven (not regularly spaced), to be defined on thetread, for the purpose of reducing the tire noise to the greatestdegree, as previously stated.

[0052] Preferably the second transverse cuts 9 have a smaller width “L′”than the first cuts 8. More particularly, the ratio between the width ofthe second cuts 9 and the width of the first cuts 8 is in the range of50% to 100%, and preferably is about 70%.

[0053] The presence of the second transverse cuts 9 in the shoulder rows5, 6, doubles the number of blocks 7, 7 a, 7 b defined therein by thefirst cuts 8.

[0054] In this connection it is apparent that the ratio between thenumber of block in two side by side block rows varies on varying of thenumber of the additional cuts (second transverse cuts 9) intermediatebetween the main cuts (first transverse cuts 8), so that said ratio canbe made different than the above-mentioned doubling, if convenient, byincreasing or decreasing the number of said additional cuts, even in adifferent manner, between the different pairs of main cuts.

[0055] Transverse slots 9 a project out as an extension of the secondcuts 9 in the blocks 7 belonging to the adjacent rows (in the case shownthe outer center rows 4) towards the equatorial plane X-X, according toa portion of lower width than that of the corresponding blocks 7.

[0056] In greater detail, each of the transverse slots 9 a projects outfor about half the axial width of the respective block 7, as anextension substantially parallel to that of the first cuts 8 whichdefine the block itself.

[0057] The presence of the transverse cuts 9 a in the blocks 7 belongingto the outer center rows 4 of course causes the number of blocks presentin said rows to be increased, so as to insure good operation of thetires, so as to achieve a balance between the number of blocks generatedin the outer center rows 4 by the presence of the first cuts 8 and thenumber of blocks generated in the inner shoulder rows 5 by the presenceof the first and second cuts 8 and 9.

[0058] One or more of the block rows, in the case shown the outershoulder rows 6, are also provided with third transverse cuts 10extending parallel to the first and second transverse cuts 8, 9 at asuitably spaced apart position therefrom. The presence of the third cuts10 in each outer shoulder row 6 substantially doubles the number ofblocks 7 a, 7 b already defined therein by the presence of the first andsecond cuts 8, 9.

[0059] In particular, it has been ascertained that in order to haveimportant and considerable advantages in a tread in accordance with theinvention, the number of blocks present in the inner center row 3 mustpreferably be between 40 blocks maximum and 20 blocks minimum around thecircumference of the tire.

[0060] As can be seen from FIG. 1, the path of the longitudinal groove 2intermediate between each of the outer 6 and inner 5 shoulder rows isinterrupted according to an alternating sequence, at the end of each ofthe third cuts 10. Therefore, each of the outer shoulder rows 6 haslinked blocks 7 a formed of a one piece construction with the blocks 7belonging to the adjacent inner shoulder row 5 alternating with freeblocks 7 b which are at least partly disengaged from the blocks 7.

[0061] The third transverse cuts 10 preferably have a width L″ which issmaller than that of the first and second transverse cuts 8, 9. In apreferential embodiment, each of the third cuts 10 along its axialextension has an inner portion 10 a projecting through the equatorialplane X-X the width of which is preferably lower than 1 mm and an outerportion, the width of which is greater than that of the inner portion 1a.

[0062] In a known manner, the first, second and third transverse cuts 8,9, 10 are also distributed in such a manner that the blocks 7 defined onthe circumferential extension of the tread 1 have differentiated sizesalternating in a predetermined sequence, usually referred to as “steppedsequence”, so that the acoustic energy produced by the tire rolling onthe road bed is distributed over a wide frequency range.

[0063] As previously stated, the distribution of the above describedfirst, second and third transverse cuts 8, 9, 10 is such that it enablesthe number of the blocks 7 associated with the individual center rows 3,4 and shoulder rows 5, 6 to gradually increase as one moves axially awayfrom the equatorial plane X-X towards the side edges 1 a of the tread 1.

[0064]FIG. 3 graphically shows the variation in the number of blocks 7in the different rows 3, 4, 5, 6 defined on the tread width. In FIG. 3,the lines denoted by A, B, C, D, E on the abscissa axis “a” correspondto width portions of the tread 1 comprised of the circumferential ribs11, the inner center rows 3, outer center rows 4, inner shoulder rows 5and outer shoulder rows 6. Numerically reproduced on the ordinate axis“b” are the number of blocks arranged in the individual areas A, B, C,D, E, in relation to the number of the blocks provided in the innercenter rows 3 corresponding to areas B.

[0065] The chain lines C′ refers to areas C to show the number of blocks7 present in the outer center rows 4 by the effect of the first cuts 8and slots 9 a.

[0066] Advantageously, on varying the number of blocks from one row tothe other, the longitudinal stiffness present at corresponding areas ofthe tread width varies correspondingly. As the graph in FIG. 2clarifies, such a longitudinal stiffness gradually increases from theaxial outer side edges 1 a of the tread 1 towards the equatorial planeX-X, with its maximum value being reached at the circumferential ribs11.

[0067] In accordance with the invention, such a variation of thelongitudinal stiffness has been found to enable the rolling noiseproduced by the tire with which the tread 1 is associated to be greatlyreduced as compared with the known art, reference being particularlymade to the noise transmitted to the outside of the motor vehicle.

[0068] In this connection it has been found that the tread areas thatare most liable to produce the outer noise are those closest to theequatorial plane X-X. In particular, at said areas the noise produced byrolling is particularly enhanced by rubbings and deformations undergoneby the blocks during the transitory steps of contacts and separationswith and from the road bed at the contact patch.

[0069] For a better explanation of why rubbings tend to be greater atthe inner areas of the tread, the following considerations should benoted.

[0070] One of the factors determining the importance of the rubbingsundergone by the blocks is their contact pressure at the contact patch.

[0071] It has been found that above all in the exit area from thecontact patch, where the separation of the blocks from the road occurs,the contact pressure exerted by the blocks disposed close to theequatorial plane is much lower than that detectable in the blocks closeto the outer edges 1 a of the tread 1. Therefore, the blocks 7 disposedclose to the center areas will tend to undergo stronger rubbings thanthose close to the side edges 1 a of the tread 1.

[0072] Another contributory factor in generating rubbings is representedby the acceleration undergone by the blocks 7 during the contact andseparation steps with and from the contact patch. As the value of suchaccelerations is a function of the square of the radius according towhich the blocks 7 are distributed about the tire axis, the blockscloser to the equatorial plane X-X are those that undergo the greatestacceleration because the tread 1, as known, has a convex outer profileas viewed in cross-section.

[0073] By virtue of the invention therefore it is possible to obviatethe greater tendency to rubbing exhibited by the blocks 7 disposed inthe areas close to the equatorial plane X-X by giving said blocks agreater longitudinal stiffness resulting from the bigger sizes of thelatter, which in turn directly result from the smaller amount of blocksdistributed over the circumferential extension of the tread in thecenter rows 3, 4.

[0074] It is to be advantageously noted that the smaller number ofblocks in the center areas of the tread 1 does not greatly impair thetraction of the tire. Traction in fact mostly relies on the great numberof blocks 7, 7 a, 7 b arranged in the shoulder rows 5, 6 where thegreatest contact pressure and smallest tendency to rubbing are found.

[0075] Also the variation in the inclination of the transverse cuts 8 isadvantageous for the purpose of reducing the outer rolling noise withoutjeopardizing the traction. In fact, the reduced angle “α” formed withthe cuts 8 and longitudinal grooves 2 close to the center rows 3, 4greatly smooths the impact of the blocks 7 onto the ground, exactly atthe areas on which the outer noise most depends. To the contrary, thecuts 8 take an orientation perpendicular to the longitudinal grooves 2and therefore to the running direction at the areas close to the sideedges where the outer noise effects are felt the least and the tractionis of great importance.

[0076] Obviously, many modifications and variations may be made to theinvention as above described, all of them falling within the scope ofthe invention as defined by the attached claims.

[0077] In particular, the tread pattern shown in FIG. 1, referringparticularly to the transverse grooves, is symmetric but said tread mayalso have an asymmetric pattern or a pattern showing mirror images onthe two sides of the equatorial plane X-X.

[0078] Obviously in manufacturing the tread of the invention thepossibility of adopting other expedients already provided by the knownart for reducing the rolling noise is not excluded.

1. A tire for motor-vehicle wheels provided with a tread producing a lowrolling noise, comprising: a raised pattern formed of a plurality ofshaped blocks distributed in parallel circumferential rows bounded bylongitudinal grooves extending circumferentially of the tire each rowcomprising a plurality of shaped blocks circumferentially separated fromeach other by respective transverse cuts with each of said rows having anumber of shaped blocks greater than the number of shaped blocks presentin the adjacent axially inner row towards the equatorial plane of thetread, the arrangement being such to give the tread a graduallyincreasing longitudinal stiffness from the axial outer side edgesthereof towards said equatorial plane.
 2. A tire according to claim 1,in which the number of said rows of blocks on each axial side of thecenter line of the tread is not smaller than three and not greater thanfive.
 3. A tire according to claim 1, in which at least one continuouscircumferential rib is disposed close to the equatorial plane, said ribbeing defined between two of said longitudinal grooves.
 4. A tireaccording to claim 3, in which there are two of said circumferentialribs disposed symmetrically with respect to the equatorial plane of thetread and mutually separated by a central groove.
 5. A tire according toclaim 1, in which the number of said shaped blocks in the axiallyinnermost row is in the range of 20 to
 40. 6. A tire according to claim1, in which the number of said shaped blocks in one row is double thenumber of shaped blocks in an axially inner adjacent row.
 7. A tireaccording to claim 1, in which in each row of said shaped blocks thetransverse cuts define an impact angle with the adjacent longitudinalgrooves, the value of said angle being greater than the value of animpact angle formed with the transverse cuts of the adjacent row ofblock disposed axially inward thereof towards the equatorial plane.
 8. Atire according to claim 7, in which the value of the impact angle in theblock rows that are closest to the equatorial plane is in the range of15° to 25°.
 9. A tire according to claim 7, in which the value of theimpact angle in the rows of blocks closest to each of the axial outerside edges of the tread is in the range of 70° to 105°.
 10. A tireaccording to claim 1, in which each of said rows has first transversecuts which are in alignment with one of the first transverse cuts of anadjacent block row.
 11. A tire according to claim 10, in which saidaligned first cuts define continuous transverse grooves each extendingfrom an outer side edge of the tread to a position adjacent saidequatorial plane.
 12. A tire according to claim 10, in which each ofsaid transverse grooves, starting from an outer side edge has onestraight portion substantially perpendicular to the longitudinalgrooves, a curved union portion and a second straight portion disposedobliquely relative to said longitudinal grooves.
 13. A tire according toclaim 11, in which said longitudinal grooves have a smaller width thanthe first transverse cuts.
 14. A tire according to claim 12, in which aratio between a width of the longitudinal grooves and a width of thefirst transverse cuts ranges between 0.6 and 0.2.
 15. A tire accordingto claim 10, in which at least one of said rows of blocks has secondtransverse cuts each being intermediate between two of said first cuts.16. A tire according to claim 15, in which the second transverse cutsare associated with outer shoulder rows, adjacent to the outer sideedges of the tread, and inner shoulder rows disposed adjacent to theouter shoulder rows towards the equatorial plane.
 17. A tire accordingto claim 15, in which the width of the second cuts is smaller than thewidth of the first cuts.
 18. A tire according to claim 17, in which aratio between a width of the second cuts and a width of the first cutsranges between 0.5 and
 1. 19. A tire according to claim 15, in which theshaped blocks of at least one of said rows each have at least onetransverse slot extending towards said equatorial plane over a portionof a smaller width than the width of that block.
 20. A tire according toclaim 19, in which said transverse slots are associated with outercenter rows disposed adjacent the inner shoulder rows towards theequatorial plane of the tread.
 21. A tire according to claim 15, inwhich at least one of said rows of blocks further comprises thirdtransverse cuts extending parallel to said first and second transversecuts.
 22. A tire according to claim 21, in which said third transversecuts are associated with outer shoulder rows adjacent to the outer sideedges of the tread.